Get Blender files to save with a thumbnail preview?
Kinda Late but here's what I found...

I ran into this post about 4 months ago with the same problem you had, after some googling and some realization that this was not currently possible I decided to change my direction of googling.
With the little knowledge of Xcode and Obj-C I had I decided to attempt to slap this together...

Please note:
It's not as err &quot;optimized&quot; as a well seasoned coder may make it, but it's enough to do the job.
So far I have had pretty good luck with it and I hope you fellows do to
Also I've found that the thumbnails that blender makes are low-quality, so its not as pretty as I hoped it would be.
And lastly i've noticed that it does not allways succeed in creating icons for the desktop (odd).

Installation
It's quite easy, download the QuickLookViewer here and move it to either ~/Library/Quicklook or /Library/Quicklook if the folder Quicklook does not exist there feel free to create it. Please notice that the one location is local in your account and the other is global on the whole computer.
Next you need to update the Quicklook engine...
Either log out and log back in (the hard solution I might add).
Or simply type the following in Terminal and hit the return key
qlmanage -r

This will force the ql engine to reset...
last of all cheers and good luck!
I guess this is my contribution to the Blender project for now! (until they implement it later cause they should)